86 INFECTION 



contact with external objects, various microorganisms, and particularly 

 the pathogenic cocci, may gain entrance to the dermis. Wounds may 

 be infected by the teeth and secretions of animals, or by various weapons 

 and implements contaminated with infective material, as, e. g. } the virus 

 in the saliva of rabid dogs, or the spores of the tetanus bacillus on rusty 

 nails. 



Contact with unclean objects of various kinds eating utensils, 

 catheters, syringes, dental instruments, etc. may serve to transfer 

 pathogenic bacteria from one person to another. This is especially 

 likely to occur if the skin or mucous membrane is abraded, the infecting 

 parasites thus gaining ready access to the deeper tissues. In some in- 

 fections, however, even this local injury is unnecessary, as the bacterium 

 may be able to proliferate and produce lesions on an intact surface, as, 

 for instance, the diphtheria bacillus in the pharynx, and various fungi, 

 such as Achorion, Trichophyton, etc., on the scalp and skin in general. 



Microparasites affecting the genital organs are likely to be conveyed 

 directly from one sex to the other in conjugation, or to the child during 

 parturition. 



4. Suctorial insects may serve as the medium by which micro- 

 organisms are transmitted from person to person. In most instances the 

 transmission is a purely mechanical process, as witness the transmissions 

 of the plague bacillus in the intestinal contents of the rat flea; in the 

 case of malaria, on the other hand, the interposition of the mosquito is 

 essential to complete the life cycle of the protozoon. 



5. Microorganisms infecting the placenta may pass to the fetus by 

 way of the umbilical vein. 



Endogenous infections arise as the result of the activity of micro- 

 organisms having their normal or customary habitat in the body. Such 

 infections do not represent so much an assumption of pathogenic power 

 on the part of the microorganism, as they do a disturbance of the de- 

 fensive mechanism of the host, whereby the normal relations are dis- 

 turbed, and microorganisms that normally are harmless, become in- 

 fective and disease-producing. While the disturbance of the defensive 

 mechanism may be general, it is far more likely to be local ; an example 

 is that of appendicitis the result of Bacillus coli infection following 

 passive congestion due to fecal impaction of the colon. 



AVENUES OF INFECTION 



Local infection may occur in any portion of the body, and any part 

 may prove the point of entrance of bacteria to the body fluids, the result 



